50 THE APODID^ PART I 



grounds ; an integumental fold, developing outwards 

 into a limb in order to fulfil various functions, would 

 require to be provided with sensory organs. Its 

 efficiency could in fact only be secured by its being 

 provided with a fine sense of touch. It is true that in 

 Apus the ventral edge of the dorsal parapodium has 

 itself developed organs of touch, viz., the endites. 

 The tip of the parapodium might also have developed 

 its own sensory organs. Economy, however, would 

 certainly lead to the use of whatever sensory organs 

 happened to be already present. 



This homology of the flabellum of Apus with the 

 sensory cirrus of the Annelidan parapodium, finds 

 some support from a study of its finer structure. Its 

 fringe of sensory hairs, its numerous ganglion cells and 

 conspicuous nerve fibres, show it at once to be a 

 sensory organ. Its flattened shape presents no diffi- 

 culty, since it is clearly thus modified to help the 

 animal in swimming. Packard thinks that it takes 

 a special share in respiration, but this we do not 

 believe, for the gills, in their inner structure and in 

 their freedom from hairs, show that they are specialised 

 for that purpose. 



Further, the consequent homology of the exopodite 

 of the typical Crustacean limb with the sensory cirrus 

 of the dorsal parapodium of the Annelida, receives 

 considerable support from a study of any series of 

 Crustacean limbs, such, for instance, as those given in 

 Lang's Text-book of Comparative Anatomy. It is im- 

 possible to avoid the impression that as a rule (with 

 no doubt many exceptions) the exopodite is a sensory 



